As a result of the demand for energy savings and the more compact design of the engine compartment of a passenger vehicle, the temperature level in the engine compartment has increased as compared to the earlier engine compartments. The increased temperature level also means that the operational temperature level of the power transmission belt has increased.
A conventional power transmission belt is formed mainly from natural rubber, styrene-butadiene rubber, or chloroprene rubber and has a hardened portion of a compressed rubber layer. The compressed rubber layer receives deformative forces during the running of the belt. However, in a high temperature environment the hardened portion cracks after a relatively short time period which can shorten the useful life of the belt.
Improvement of the heat resistance of chloroprene rubber has been achieved to a certain degree. However, use of chloroprene rubber itself limits the amount of improvement that can be achieved. Therefore, satisfactory improvement has yet to be made.
In view of the above facts, studies are being undertaken in the use of rubber materials whose principal chain is highly or completely saturated. Representative rubber material include chlorosulfonated polyethylene rubber, hydrogenated acrylonitrile-butadiene rubber, fluororubber, and the like, all of which have excellent heat resistance. Among these rubber materials, it is known that chlorosulfonated polyethylene generally has the same dynamic fatigue resistance, abrasion resistance, and oil resistance as chloroprene rubber. However, the water resistance of chlorosulfonated polyethylene is highly influenced by the vulcanizing substance, particularly the acid accepter.
Generally, oxide materials such as MgO or PbO have been conventionally used as an acid accepter for chlorosulfonated polyethylene to react with HCl produced during chlorosulfonating to produce MgCl.sub.2 and water and PbCl.sub.2 and water, respectfully. Although use of a lead compound such as PbO or Pb.sub.3 O.sub.4 as an acid accepter can achieve the production of a belt having good water resistance, the use of such lead compounds is not desirable in terms of incurring environmental pollution or sanitary problem. When MgO is used as an acid accepter, MgCl.sub.2 generated during the crosslinking reaction process significantly reduces water resistance of the resulting product which means utilization of MgO in a belt is undesirable.
When an epoxy system acid accepter is used instead of a metal oxide, a composition having superior water resistance can be obtained. Unfortunately the epoxy system produces an unpleasant odor.
As a solution to the above-mentioned problems it has been proposed in Japanese Patent Laid-open No. 62-246951 to make a power transmission belt made of a chlorosulfonated polyethylene rubber-containing composition containing a magnesium oxide-aluminum oxide solid solution as an acid accepter at least in the compressed rubber layer of the power transmission belt. This power transmission belt has a longer service life in a high temperature environment as compared with a chloroprene rubber belt and exhibits excellent heat resistance. Unfortunately, the chlorosulfonated polyethylene rubber belt has a shorter service life when utilized at a temperature not higher than -30.degree. C. The reason for the shorter low temperature service life is presently believed to be because the conventional chlorosulfonated polyethylene (referred to as CSM) is formed by chlorosulfonating a straight-chain high density polyethylene usually having a density of 0.946 to about 0.970 grams per cubic centimeter to produce a CSM having a chlorine content of 35% by weight. This relatively high chlorine content is presently believed to maintain the rubber resiliency by disrupting the polyethylene crystals which results in increasing the chlorine cohesion energy to harden the rubber itself at a low temperature causing lower rubber resiliency and the formation of cracks.
A power transmission belt with improved operation durability and increased service life in both high and low temperature environments due to improving the composition in the compressed rubber layer to develop high and low temperature resistance of the compressed rubber layer is desirable.